one girl's diary of improvisational engineering

Self-contained TED receiver

My previous entry introduced a homebrew receiver for the powerline-based data protocol used by The Energy Detective. I just designed a second revision of that receiver. This one is self-contained: It gets power and modulated data from a 9V AC wall-wart transformer, and decoded data leaves via an RS-232 serial port at 9600 baud. Best of all the circuit is very simple: Just an 8-pin microcontroller and a single op-amp.

Major changes in this version:

DC power for the circuit is now provided by the 9V AC input, instead of a separate power supply. Previously this would have caused unacceptable levels of harmonic distortion in the input signal. In the new design, this is mitigated by an inductor (which forms an LC low-pass filter), and by the lower power consumption of a single modern op-amp versus three ancient op-amps.

By using a simpler filter design and a modern op-amp with a gain-bandwidth product of at least 10 MHz, the bandpass filter and amplifier can be built using only a single op-amp.

Note that the MAX475 op-amp I’m using has been discontinued by the manufacturer, and it’s now hard to find. I just used it because I had one handy in my junk drawer. I’ll verify this design with other op-amps as soon as I can, but it should work with just about any op-amp which can operate on a single-ended 5V supply, and which has a high enough GBW.